An echo canceller device is used in a loudspeaker calling device or the like, for the purpose of cancelling an echo of a speech reception signal that is mixed in a signal input to a microphone (hereinafter, referred to as an input signal), from the input signal. As described in Non-patent Literature 1, in many cases, the echo canceller device used for this purpose each is constituted of an adaptive filtering unit and a residual echo suppression unit. FIG. 5 illustrates a schematic diagram of the echo canceller device having this constitution.
An adaptive filtering unit 300 estimates (learns) an echo path h, generates a filter coefficient ĥ and a pseudo echo signal (echo replica), and cancels from an input signal input to a microphone.
Nevertheless, an echo that can be cancelled by the adaptive filtering unit 300 is approximately 30 dB, and all echo signals cannot be cancelled. Thus, a residual echo suppression unit 400 is often included in order to suppress residual echoes.
Nevertheless, residual echo suppressing processing simultaneously suppresses a speech transmission signal, for example, a signal of voice spoken to a microphone. Thus, a large suppression amount may have a negative influence on the quality of bidirectional simultaneous telephone call (double-talk) performed by a near-end speaker and a far-end speaker. For the purpose of enhancing the performance of the double-talk, it is effective to increase an amount of echoes cancelled by an adaptive filter and decrease the suppression amount of residual echo suppression as little as possible.
On the other hand, in handsfree telephone call, the power of an echo signal in a low-frequency band is often strong. This is attributed to the fact that the low-frequency band of a speech signal has strong power, and to the design of speakers and echo characteristics in a vehicle. Residual echo signals, which are residual signals remaining after echoes having the aforementioned feature have been cancelled with the adaptive filter, are likely to remain in the low-frequency band. FIG. 6 illustrates an example of input signals recorded in a vehicle. All of the input signals illustrated in FIG. 6 are assumed to be echo signals. FIG. 7 illustrates average powers at respective frequencies of an echo signal (input signal) illustrated in FIG. 6, and a residual signal remaining after the echo signal illustrated in FIG. 6 has been cancelled with an adaptive filter, i.e., a residual echo signal.
As illustrated in FIG. 7, it can be seen that a speech transmission signal has a peak of power in the vicinity of 500 Hz, and a residual echo signal similarly has large power in the vicinity of 500 Hz. If double-talk occurs in this vehicle, it is necessary to apply strong residual echo suppression on the vicinity of 500 Hz, for reducing the residual echo signal to an inaudible level for users. However, in the case where the residual echo suppression is applied, strong suppression is applied also to the vicinity of 500 Hz of the speech transmission signal, leading to deterioration in quality of double-talk.
In the speech transmission signal, as for a high-frequency band having small power, if a large scale of echo reduction is not performed with the adaptive filter, but echo reduction is performed by subsequent residual echo suppression, telephone transmission voice is not negatively affected. Nevertheless, as for the low-frequency band of the speech transmission signal, double-talk quality can be enhanced by reducing more echoes with the adaptive filter.
Thus, in an acoustic echo canceller disclosed in Patent Literature 1, the speech reception signal and the speech transmission signal are divided into a plurality of bands, and influence of an echo in each band is estimated, thereby determining a priority order. In each band, the adaptive filter determines processing based on the priority order. The estimation of influence of the echo is performed in such a manner that a higher priority order is allocated to a band in which echo feeling is considered to be acoustically felt the most. An adaptive filter in a high-priority band performs complicated processing with a large echo suppression effect, and an adaptive filter in a low-priority band performs simple processing. By determining a priority order in accordance with a band as described above, echo reduction processing, in which an acoustic characteristic is considered, is performed, and telephone call of high quality is provided.